Viral hemorrhagic fever is a serious illness characterized by extensive vascular damage and bleeding diathesis, fever, and multiple organ involvement. Many different viruses can cause this syndrome, each with its own animal reservoir, mode of transmission, fatality rate, and clinical outcome in humans. These viruses are distributed throughout four virus families, the Arenaviridae, Bunyaviridae, Filoviridae, and Flaviviridae. Several of these viruses generate significant morbidity and mortality and can be highly infectious by aerosol dissemination, promoting concern about weaponization (for an overview, see 3). In 1999, the Centers for Disease Control and Prevention (CDC) identified and categorized potential biological terrorism agents as part of a Congressional initiative to upgrade bioterrorism response capabilities (30). Filoviruses and arenaviruses were designated as Category A, defined as those pathogens with the highest potential impact on public health and safety, potential for large-scale dissemination, capability for civil disruption, and greatest unmet need for public health preparedness. The National Institute of Allergy and Infectious Diseases (NIAID) has since expanded the Category A list by adding several hemorrhagic bunyaviruses and flaviviruses (27). In addition, the Working Group on Civilian Biodefense described several hemorrhagic fever viruses, including Lassa, as those with the greatest risk for use as biological weapons and recommended the pursuit of new antiviral therapies (3).
Prevention and treatment options for hemorrhagic fever viruses are limited. With the exception of an effective vaccine for yellow fever, no licensed vaccines or FDA-approved antiviral drugs are available. Intravenous ribavirin has been used with some success to treat arenaviruses and bunyaviruses, although its use has significant limitations (see below). In addition, there have been recent reports of promising vaccines for Ebola (19) and Lassa (16). Although a successful vaccine could be a critical component of an effective biodefense, the typical delay to onset of immunity, potential side-effects, cost, and logistics associated with large-scale civilian vaccinations against a low-risk threat agent suggest that a comprehensive biodefense include a separate rapid-response element. Thus there remains an urgent need to develop safe and effective products to protect against potential biological attack.
Lassa fever virus is a member of the Arenaviridae family, a family of enveloped RNA viruses (4). Arenavirus infection in rodents, the natural host animal, is usually chronic and asymptomatic. Several arenaviruses can cause severe hemorrhagic fever in humans, including Lassa, Machupo, Guanarito, and Junin viruses. Transmission to humans can result from direct contact with infected rodents or their habitat, through aerosolized rodent secretions, or through contact with the body fluids of an infected person. Although arenaviruses are found world-wide, most of the viral species are geographically localized to a particular region, reflecting the range of the specific rodent host involved. The Arenaviridae family contains a single genus (Arenavirus) that is divided into two major lineages based on phylogenetic and serological examination. Lassa fever is a member of the Old World arenaviruses; the New World arenaviruses can be further divided into three clades (A-C), one of which (clade B) contains several of the pathogenic, Category A hemorrhagic fever viruses.
Lassa fever is endemic in West Africa, particularly the countries of Guinea, Liberia, Sierra Leone, and Nigeria. Human infections are estimated at 100,000 to 500,000 per year (25). Initial symptoms of Lassa fever appear about 10 days after exposure, and include fever, sore throat, chest and back pain, cough, vomiting, diarrhea, conjunctivitis, facial swelling, proteinuria, and mucosal bleeding. Clinical diagnosis is often difficult due to the nonspecific nature of the symptoms. In fatal cases, continuing progression of symptoms leads to the onset of shock. Among hospitalized patients, the mortality rate is 15-20% (23), although the fatality rate for some outbreaks has been reported higher than 50% (14). Infectious virus can remain in the bodily fluids of convalescent patients for several weeks (34). Transient or permanent deafness is common in survivors (10) and appears to be just as frequent in mild or asymptomatic cases as it is in severe cases (22). Lassa fever is occasionally imported into Europe (17) and the U.S., most recently in 2004 (7). The risk of the virus becoming endemic outside of West Africa appears low due to the nature of the rodent host. However, the combination of increased world travel and viral adaptation presents a finite possibility of a virus “jumping” into a new ecosystem. For example, West Nile virus was introduced into the New York City area in 1999 and is now endemic in the U.S.
A small trial conducted in Sierra Leone in the 1980s demonstrated that mortality from Lassa fever can be reduced in high-risk patients by treatment with intravenous ribavirin, a nucleoside analog that exhibits nonspecific antiviral activity (24). Ribavirin has been shown to inhibit Lassa fever viral RNA synthesis in vitro (18). Although of limited availability, intravenous ribavirin is available for compassionate use under an investigational new drug protocol. It is also available in oral form for treating hepatitis C (in combination with interferon), although less is known about the efficacy of orally-administered ribavirin for treating Lassa fever. As a nucleoside analog, ribavirin can interfere with DNA and RNA replication, and in fact teratogenicity and embryo lethality have been seen in several animal species. It is therefore contraindicated for pregnant patients (a pregnancy category X drug). In addition, it is associated with a dose-related hemolytic anemia; although the anemia is reversible, anemia-associated cardiac and pulmonary events occur in approximately 10% of hepatitis C patients receiving ribavirin-interferon therapy. Intravenous ribavirin is expensive, and daily I.V. administration to a large civilian population in an emergency would be a cumbersome approach. It is possible that further study may eventually support the use of oral interferon, either alone or in combination with other antivirals, for treatment of Lassa fever. Successful antiviral therapy often involves administering a combination of pharmaceuticals, such as the treatment of chronic hepatitis C with interferon and ribavirin, and treatment of AIDS with highly active antiretroviral therapy (HAART), a cocktail of three different drugs. Because of the high mutation rate and the quasispecies nature associated with viruses, treatment with compounds that act on multiple, distinct targets can be more successful than treatment with a single drug.
The arenavirus genome consists of two segments of single-stranded RNA, each of which codes for two genes in opposite orientations (referred to as ambisense). The larger of the two segments, the L RNA (7.2 kb), encodes the L and Z proteins. The L protein is the RNA dependent RNA polymerase, and the Z protein is a small zinc-binding RING finger protein which is involved in virus budding (29). The S RNA (3.4 kb) encodes the nucleoprotein (NP) and the envelope glycoprotein precursor (GPC).
The envelope glycoprotein is embedded in the lipid bilayer that surrounds the viral nucleocapsid. The characteristics of the arenavirus glycoprotein suggest that it can be classified as a Type I envelope (15), which is typified by influenza hemagglutinin and found also in retroviruses, paramyxoviruses, coronaviruses, and filoviruses (8). Type I envelopes function both to attach the virus to specific host cell receptors and also to mediate fusion of the viral membrane with the host membrane, thereby depositing the viral genome inside the target cell. Cotranslational translocation of the envelope protein across the membrane of the endoplasmic reticulum is facilitated by an N-terminal signal peptide that is subsequently removed by a signal peptidase. Post-translational proteolysis further processes the envelope into an N-terminal subunit (denoted GP1 for arenaviruses), which contains the receptor binding determinants, and a C-terminal transmembrane subunit (GP2), which is capable of undergoing the dramatic conformational rearrangements that are associated with membrane fusion. The two subunits remain associated with one another and assemble into trimeric complexes of this heterodimer, although arenavirus envelope glycoproteins have been reported to have a tetrameric structure (5). Mature envelope glycoproteins accumulate at the site of viral budding, such as the plasma membrane, and thus are embedded within the envelope that the virus acquires as viral budding occurs.
The signal peptide of the arenavirus glycoprotein is quite unusual (12); at 58 amino acids in length, it is larger than most signal peptides (13). In addition, it remains associated with the envelope and with mature virions, and appears to be important for the subsequent GP1-GP2 processing (11). This processing is essential for envelope function and is mediated by the cellular subtilase SKI-1/S1P (1, 20, 21). The envelope glycoprotein interacts directly with the host cellular receptor to facilitate viral entry into the target cell. The receptor for Old World arenaviruses is α-dystroglycan (6), a major component of the dystrophin glycoprotein complex. The New World arenaviruses appear to have diverged from this receptor, as only the Glade C viruses use α-dystroglycan as a major receptor (32). The receptor for the New World clades A and B arenaviruses has not yet been identified.